1. Field of the Invention
The present invention relates to an apparatus and method for processing substrates. Specifically, the invention relates to a method for depositing conformal barrier layers and seed layers in an ionized metal plasma process.
2. Background of the Related Art
Sub-quarter micron multi-level metallization represents one of the key technologies for the next generation of ultra large-scale integration (ULSI) for integrated circuits (IC). In the fabrication of semiconductors and other electronic devices, directionality of particles being deposited is important in filling small features. As circuit densities increase, the widths of vias, contacts and other features have decreased to 0.25 .mu.m or less, whereas the thicknesses of the dielectric layers remain substantially constant. Thus, the aspect ratios for the features, i.e., the ratio of the depth to the minimum lateral dimension, increases, thereby pushing the aspect ratios of the contacts and vias to 5:1 and above. As the dimensions of the features decrease, it becomes even more important to get directionality of the particles in order to achieve conformal coverage of the feature side walls and bottom.
Conventional physical vapor deposition (PVD) are not suited for directional deposition and, therefore, have difficulty penetrating and conformally lining the sidewalls and bottoms of structures where the aspect ratio exceeds 4:1. Thus, the uniformity and step coverage of the deposited layer will depend directly upon the structure architecture with the layer becoming thinner on the structure bottom and sidewall near the bottom. The uniformity and step coverage of the layer, and therefore the integrity of the layer, will be compromised by overhangs, voids and other undesirable formations in the device features.
To obtain deposition in the high aspect ratio features, one method uses a medium/high pressure physical vapor deposition (PVD) process known as an ionized metal plasma (IMP) process or high density plasma physical vapor deposition (HDP-PVD). The plasma density in IMP processes are typically between about 10.sup.11 cm.sup.-3 and 10.sup.12 cm.sup.-3. Generally, IMP processing offers the benefit of highly directional deposition with good bottom coverage in high aspect ratio structures. Initially, a plasma is generated by introducing a gas, such as helium or argon, into the chamber and then biasing a target to produce an electric field in the chamber, thereby ionizing a portion of the gas. An energized coil positioned proximate the processing region of the chamber couples electromagnetic energy into the plasma to result in an inductively-coupled medium/high density plasma between the target and a susceptor on which a substrate is placed for processing. The ions and electrons in the plasma are accelerated toward the target by the bias applied to the target causing the sputtering of material from the target. Under the influence of the plasma, the sputtered metal flux is ionized. An electric field due to an applied or self-bias, develops in the boundary layer, or sheath, between the plasma and the substrate that accelerates the metal ions towards the substrate in a direction substantially parallel to the electric field and perpendicular to the substrate surface. The bias energy is preferably controlled by the application of power, such as RF, to the susceptor to attract the sputtered target ions in a highly directionalized manner to the surface of the substrate to fill the features formed on the substrate.
The high density plasma of conventional HDP-PVD is typically achieved by operating at pressures between about 5-100 mTorr. It is believed that such pressures ensure thermalization and ionization of the sputtered metal particles. Thermalization refers to the slowing of the metal particles passing through the plasma by collisions with the plasma ions and must be sufficiently high to allow time for the coil to ionize the metal particles. Should the metal particles travel from the target to the substrate too quickly, the metal particles may not be ionized resulting in poor deposition rates and uniformity.
In an attempt to increase thermalization and ionization of the sputtered metal particles, it has been suggested to increase the chamber pressure, thereby increasing the plasma density. The higher plasma density, in turn, reduces the mean free path between particles, resulting in more collisions and increased ionization. However, above a certain pressure the deposition results are compromised. In particular, because of the greater number of collisions, the metal particles lose their initial directionality from the target and, in fact, may be back-scattered onto the target or other chamber components, thereby decreasing the deposition rate. Even those particles which continue toward the substrate may strike the device features at an angle oblique to the surface, despite the bias applied to the substrate, resulting in poor step coverage at the structure bottom and the side walls at the structure bottom.
Another problem related to higher operating pressures is the resulting low plasma potential. In order to bias ions toward the substrate for deposition thereon, a voltage, or potential, must be applied to the substrate. The voltage (V) is typically supplied by an RF or DC power supply, as described above, and is related to power (P) and current (I) according to V=P/I. As the plasma becomes denser at higher pressures, the current increases, thereby reducing the voltage applied to the substrate at a constant power level. To increase the voltage to a desired level, the power to the substrate must be increased. However, excessive power can damage the substrate by overheating, thereby preventing the power level from exceeding a critical value. Thus, the plasma density must be low enough to ensure a sufficiently high plasma potential and high bias on the substrate.
A different problem with conventional HDP-PVD is the emission profile, or directionality, of the sputtered target material from the target which, in part, determines the step coverage. Sputtering of the material from the target follows distribution patterns ranging from under-cosine to cosine to over-cosine. FIGS. 1-3 are typical under-cosine 10, cosine 12 and over-cosine emission profiles 14, respectively. Each of the emissions profiles 10, 12, 14 define the probability of a particle being sputtered from the target at a particular angle. Lines 16 emanating from arbitrary ejection point 18 indicate various ejection angles and the probability of a particle being ejected at that angle. The probability that an atom 15 will be sputtered from the target 17 at a specific angle is related to the length of the lines 16 originating at the ejection point 18. For example, in FIG. 1, the length of A is 1.3 times that of B, indicating the probability of the ejected atom 15 having trajectory 30 degrees relative to the plane of the target 17 (i.e., along the line B) is 0.77 times (1/1.3) that of the atom going out orthogonally (i.e., along line A). Thus, the overcosine emission profile 14 shown in FIG. 3 provides the greatest bottom coverage in high aspect ratios because of the greater normal or near-normal directionality from the target. Accordingly, the over-cosine emission profile 14 is most desirable for high aspect features.
Currently, the preferred plasma gases for HDP-PVD processes are argon (Ar) and helium (He) because of their low cost. While Ar and He have proven suitable for sputtering some target materials, such as Si, Ti/TiN and Al, Ar and He do not produce desirable emission profiles for other materials such tantalum (Ta), tungsten (W) and copper (Cu). Under optimal conditions, sputtering W in an Ar or He plasma produces only a cosine emission profile. The resulting cosine emission profile negatively impacts the coverage of device features. In particular the bottoms and lower sidewalls are not conformally covered with deposition material.
Therefore, there is a need for a method of depositing material on a substrate in an inductively-coupled plasma environment wherein the resulting layers exhibit good uniformity and step coverage.